No, that's just what Ruger says. I know they work from one new model to the next.

Click to expand...

No, that's not just what Ruger says. Any competent revolver-smith will tell you that too.
The cylinder/hand/lock combo is timed at the factory to provide the proper chamber indexing. If they're not installed and timed properly you might run into lock-up problems or chambers that are misaligned to the barrel.
As well-built that the Ruger revolvers are, they're production tolerances are not tight enough that parts are 100% drop-in interchangeable between guns.
Check your cylinder/chamber alignment if you've fitted a new cylinder.

Yes, the New and Old model SS cylinders are dimensionally the same and are interchangable. The key differences between the Old and New Models are in the internal lockwork
Most cylinder swaps will most likely fit and function, but the key is will they function properly and accurately. The dual-cylinder sets are matched up & timed at the factory and are marked with the last 3 digits of the frames serial number.

Well it would be a sad thing indeed if a top notch U.S. company can't have machinery precise enough to make thousands of the same thing exactly alike so they're a drop in fit but an overseas co. like Pietta that makes cheap reproductions can. I'm very familiar with the lockwork on single actions. As long as it will fit I can handle timing.

Trust me...those "drop-in" parts for the cheap (and not so cheap too) repro companies need to be fitted and timed the same way.
I've gone through at least two dozen of them Pietta cap-n-ball clones (early and current) to correct timing problems with just one cylinder much less an extra. They don't give a whit about several thousands of an inch tolerance either way, which is enough to turn a revolver into a chunk of metal that throws shotgun groups at 15yds.

With revolver timing you're looking at, at the very least, single-digit thousands of an inch tolerance for a plinker/hunter and preferably ten-thousands tolerance levels if you want a match-grade weapon.
Compared to some other big makers Ruger has always been a little on the sloppier side of fitting from the factory to expedite the manufacturing process and trim down the amount of hand-fitting needed. Most of the companies are moving this way but that's been Ruger's mantra for many years.
They're a heckuva system that can be tightened up superbly, but out of the box darn near every one you look at is on the sloppy side and can benefit from improvement.

Either way, be sure to check out your new cylinder to see that it's indexing and locking properly with the chambers aligned with the bore.